An active energy ray-polymerizable composition which is polymerized and cured by active energy rays, particularly a so-called photopolymerizable composition which is polymerized and cured by being irradiated with ultraviolet rays or visible light, has been widely used in the field of coating, ink, electronic material, etc. This technique of curing a polymerizable composition by active energy rays including ultraviolet rays and visible light, has been used for various applications, for example, a wood coating material, a coating material for e.g. a metal, an ink for screen printing or off-set printing, an ink for an ink jet printer, a dry film resist used for an electronic substrate, a hologram material, a sealing agent, a hard coating agent, an overcoat agent, an optical element such as a lens or a lens sheet, an optical material, a resin for stereolithography, a bonding agent/adhesive.
And, such a photopolymerizable composition mainly comprises a photopolymerizable compound, a photopolymerization initiator which initiates polymerization of the photopolymerizable compound by energy irradiation, and in many cases, a photopolymerization sensitizer which activates the photopolymerization initiator. Particularly, a photoradical polymerizable composition which is polymerized and cured by employing radical polymerization mechanism, comprises a radical polymerizable compound and a radical polymerization initiator, and is industrially very widely used in view of wide range of selection of the composition, easy adjustment of physical properties of a cured product, availability of the polymerizable compound, etc.
The radical polymerization initiator is mainly classified into an intramolecular cleavage type and a hydrogen withdrawal type. In the radical polymerization initiator of the intramolecular cleavage type, by absorption of light having a specific wavelength, a bond at a specific portion is broken, radicals are generated at the broken portion, which function as a polymerization initiator to initiate polymerization of the polymerizable compound. On the other hand, in the case of hydrogen withdrawal type, the radical polymerization initiator absorbs light having a specific wavelength and is in an excited state, the excited species bring about hydrogen withdrawal reaction from a surrounding hydrogen donor and as a result, radicals are generated, which function as a polymerization initiator to initiate polymerization of the polymerizable compound.
As an intramolecular cleavage type radical polymerization initiator, an alkylphenone, an acylphosphine oxide, an oxime ester, etc. have been known (Patent Document 1). On the other hand, as a hydrogen withdrawal type radical polymerization initiator, benzophenone or a compound having a benzophenone skeleton (benzophenone type photoradical polymerization initiator), etc. have been known (Patent Document 2).
A compound having a benzophenone skeleton which is a hydrogen withdrawal type radical polymerization initiator is characterized in that it can suppress coloring of a cured product, however, the compound having a benzophenone skeleton has an absorption region of usually only about 360 nm, and polymerization does not proceed or is not sufficient if the polymerizable composition is to be polymerized and cured by light having a relatively long wavelength, and the compound is not suitable for curing by a long wavelength light. Particularly in a case where the polymerizable composition is polymerized and cured e.g. by a LED having an irradiation wavelength of at least 370 nm which becomes heavily used in recent years, a polymerized product may not be obtained, or the polymerization rate is low and physical properties of the resulting cured product are not sufficient.